Apparatus and method for driving led display panel

ABSTRACT

The present disclosure provides devices, circuits, and methods for driving an LED display panel, so as to ensure the delivery of equal global charge at minimum gray scale. In one example, the circuit of the present disclosure includes a current source, a detection module, and a measurement module. The current source is configured to generate a current signal for the LEDs. The detection module is configured to detect a forward voltage of the LEDs in response to a low current applied to the LEDs. The detection module is further configured to hold the detected forward voltage. The measurement module is configured to measure a time period for an anode voltage of the LEDs to rise to the detected forward voltage in response to a display current applied to the LEDs.

This application is a divisional application of U.S. application Ser.No. 13/684,197 filed on Nov. 22, 2012, the content of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates generally to devices, circuits, andmethods for driving light emitting diode (LED) display panels. Moreparticularly, the present disclosure relates to devices, circuits, andmethods for driving the LED display panels, so as to ensure the deliveryof equal global charge at minimum gray scale.

BACKGROUND

Recently, light emitting diodes (LEDs) have been widely used inelectronic devices and applications. For example, LEDs have been used aslight sources for general illumination. Additionally, LEDs have beenused to make display panels, televisions, etc. Regardless of theapplications, driving circuits are required to supply power to the LEDsand to control the LEDs to illuminate light with the desired brightness.

An LED display panel generally refers to a device which comprises anarray of LEDs that are arranged in one or more rows and columns.Alternatively, an LED display panel may include a plurality ofsub-modules, each sub-module having one or more such LED arrays. LEDpanels may employ arrays of LEDs of a single color or different colors.When LEDs of the same color are used in certain display applications,each LED normally corresponds to a display unit or pixel. When LEDpanels employ LEDs of different colors, a display unit or pixel normallyincludes a cluster of three LEDs, which may include a red LED, a greenLED, and a blue LED. Such a cluster of three LEDs may be referred to asan RGB unit. Surface mounted RGB units usually have four pins. Thefirst, second, and third pins may respectively correspond to the red,green, and blue LEDs. The fourth pin may correspond to either a commonanode or a common cathode of the LEDs.

An LED driving circuit delivers power to the array of LEDs and controlsthe current delivered to the array of LEDs. The driving circuit may be asingle channel driver or a multi-channel driver. Each channel of thedriving circuit may deliver power to a plurality of LEDs and control thecurrent delivered to the LEDs. When a group of LEDs is electricallycoupled to the same channel, the group of LEDs are often referred to asa “scan line.”

In general, LED driving circuits control the brightness of the LEDs byvarying the current delivered to and flowed through the LEDs. Inresponse to the delivered current, the LED emits light with a brightnessin accordance with the characteristic specifications of the LED. Agreater current delivered to the LED usually translates to a greaterintensity of brightness. To effectively control the delivery of current,LED driving circuits may employ a constant current source in combinationwith the modulation (i.e., turning ON and OFF) of the constant current,using, for example, Pulse Width Modulation (PWM).

FIG. 1A illustrates an ideal PWM signal 110 having a width W and anamplitude A for each PWM cycle. By varying width W of PWM pulse 110, theLED driving circuits may effectively deliver proper driving currents tothe LEDs, so as to illuminate light at different shades of gray scale.When delivering the PWM signal, the driving circuit may see differentload characteristics for each LED. Such variability of loadcharacteristics may be constituted by a number of accumulating effects,such as the variation of forward voltage V_(f) for each LED, thevariation of intrinsic impedance for each scan line, and the variationof the response to forward current If for each LED. These effects causehuge variations of brightness amongst the LEDs, especially at low grayscale settings. FIG. 1B illustrates an exemplary PWM signal 120 as seenin the driving circuit due to the variability.

Further, at a low gray scale setting, width W of a PWM signal may besufficiently narrow, such that the pulses of the PWM signal may be lostdue to the uncompensated nature of the driving current and the loadcharacteristics. Accordingly, system designers have resorted to apre-emphasis methodology to overcome the distortion of the PWM signalson PCB traces.

FIG. 1C illustrates an ideal PWM signal 130 with a pre-emphasis portion135 at the beginning of each PWM cycle. In general, pre-emphasis portion135 has a duration or width D, which is usually less than width W of aregular PWM cycle, and an amplitude A′ greater than amplitude A of aregular PWM cycle. In the driving circuit for LED display panels, a PWMcurrent signal may be corrected with pre-emphasis, so as to overcome orcompensate signal distortions at the rise time. FIG. 1D illustrates anexemplary PWM signal 140 as seen in the driving circuit, withpre-emphasis.

In low gray scale settings, however, duration D of pre-emphasis portion135 may be proximate or greater than width W of a PWM cycle.Accordingly, even with pre-emphasis, the PWM driving signal may still bedistorted, resulting in short pulses and/or accompanying with PWMringing. Consequently, the PWM driving signal may require furtherprocessing, especially for the cases of low gray scale settings.

SUMMARY OF INVENTION

The present disclosure provides devices, circuits, and methods fordriving an LED display panel, so as to ensure the delivery of equalglobal charge at minimum gray scale.

In one embodiment, a method is provided for driving an LED displaypanel. The method includes initializing a plurality of LEDs of the LEDdisplay panel, and operating the LED display panel by supplying adriving signal to the LEDs so as to illuminate light with desiredbrightness. Initializing the LEDs includes determining a compensationparameter of the LEDs. Operating the LED display panel includesgenerating a pre-determined driving signal, modifying the pre-determineddriving signal of the LEDs using the compensation parameter to obtain amodified driving signal, and supplying the modified driving signal tothe LEDs.

In one embodiment, a circuit is provided for ensuring full content cyclelighting of an LED array having a plurality of LEDs. The circuitincludes a current source configured to generate a current signal forthe LEDs, a detection module configured to detect a forward voltage ofthe LEDs in response to a low current applied to the LEDs, the detectionmodule further configured to hold the detected forward voltage, and ameasurement module configured to measure a time period for an anodevoltage of the LEDs to rise to the detected forward voltage in responseto a display current applied to the LEDs.

In one embodiment, a circuit is provided for driving an LED displaypanel having a plurality of LEDs. The circuit includes an initializationunit and an operation unit. The initialization unit includes a detectionmodule and a measurement module. The detection module is configured todetect a forward voltage of the LEDs in response to a low currentapplied to the LEDs. The detection module is further configured to holdthe detected forward voltage. The measurement module is configured tomeasure a time period for an anode voltage of the LEDs to rise to thedetected forward voltage in response to a display current applied to theLEDs. The operation unit includes a signal generator configured togenerate a driving signal, a compensation module configured to modifythe driving signal in accordance with the measured time period, and acurrent driver configured to receive the modified driving signal and, inresponse to the modified driving signal, transmit a driving current tothe LEDs, so as to illuminate light with desired brightness.

DESCRIPTIONS OF DRAWINGS

The teachings of the present disclosure can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings.

FIG. 1A illustrates an ideal pulse width modulation signal.

FIG. 1B illustrates an exemplary pulse width modulation signal as seenin a driving circuit, without pre-emphasis.

FIG. 1C illustrates an ideal pulse width modulation signal with apre-emphasis portion at the beginning of each cycle.

FIG. 1D illustrates an exemplary pulse width modulation signal as seenin a driving circuit, with pre-emphasis.

FIG. 2 schematically illustrates a circuit in accordance with oneembodiment of the present disclosure.

FIG. 3 schematically illustrates a timing diagram in accordance with oneembodiment of the present disclosure.

FIG. 4 schematically illustrates a circuit in accordance with anotherembodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENT

The Figures (FIG.) and the following description relate to theembodiments of the present disclosure by way of illustration only. Itshould be noted that from the following discussion, alternativeembodiments of the structures and/or methods disclosed herein will bereadily recognized as viable alternatives that may be employed withoutdeparting from the principles of the claimed inventions.

Reference will now be made in detail to several embodiments of thepresent disclosure(s), examples of which are illustrated in theaccompanying figures. It is noted that wherever practicable similar orlike reference numbers may be used in the figures and may indicatesimilar or like functionality. The figures depict embodiments of thepresent disclosure for purposes of illustration only. One skilled in theart will readily recognize from the following description thatalternative embodiments of the structures and methods illustrated hereinmay be employed without departing from the principles of the disclosuredescribed herein.

The present disclosure relates to a device, an integrated circuit, and amethod to control pixels of an LED display panel and to ensure fullcontent cycle lighting (FCCL). Additionally, the present disclosurerelates to a device, an integrated circuit, and a method to compensatefor system related variability that causes brightness variations fromLEDs in an LED array, so as to ensure the delivery of equal globalcharge at minimum gray scale settings. These system related effects maybe accumulated and cause minimum pulses (i.e., PWM signals at low grayscale settings) to have large variations in different LED channelsand/or in different chips. Hence, it would be advantageous to know thedirect impedance and the response characteristics of each LED in thescan lines, so as to compensate for the corresponding system relatedeffects. This would guarantee that intended charge be delivered to theLED, regardless of forward voltage V_(f) specification of the LED, orthe relative position and/or consequential impedance effect of the LEDin the string of an LED array.

In general, a method for driving an LED display panel may include aninitialization step and an operation step. The initialization step mayinclude powering on the LED display panel and testing LEDs of the LEDdisplay panel; while the operation step may include selecting a scanline from a plurality of scan lines of an LED array, sequentiallysupplying electric power to each of the LEDs on the selected scan line,and repeating the selecting step and the supplying step until the LEDson all scan lines are illuminated to display a screenshot. The operationstep may be repeated for displaying motion pictures.

In one embodiment, the LED driving method of the present disclosure mayinclude a method for compensating system related variability, which maybe incorporated with the initiation step and/or the operation step ofthe LED driving method. The compensating method of the presentdisclosure may include a preparation step and a modification step. Thepreparation step of the compensating method may include a detection stepand a measurement step. The preparation step may be performedimmediately after powering on the LED display panel. Alternatively,depending on design preferences, the preparation step of thecompensating method may also be performed during the operation step ofthe LED driving method. Further, the modification step of thecompensating method may be incorporated with the operation step of theLED driving method. In one embodiment, the modification step may includeextending the width of an uncompensated PWM current signal whensupplying electric power to an LED.

FIG. 2 schematically illustrates an FCCL circuit 200 that can be used toimplement the method for compensating system related variability, inaccordance with one embodiment of the present disclosure. As shown inFIG. 2, FCCL circuit 200 includes a channel output 210, a comparator220, a three-terminal switch 230, a detection module 240, and ameasurement module 250.

Channel output 210 may receive a control signal from a control terminal202 and, in response to the control signal, deliver a display current toan LED array 260. LED array 260 includes a plurality of LEDs 264, whichmay be arranged in rows and columns. In one embodiment, plural LEDs 264may be electrically coupled to a common anode 262 to form a channel. Itis to be understood that, in other embodiments, LEDs 264 may beelectrically coupled to a common cathode. Common anode 262 may beelectrically coupled to the non-inverting input (+) of comparator 220.Comparator 220 may compare voltages at its non-inverting input (+) andinverting input (−), and send output to a first terminal ofthree-terminal switch 230.

In one embodiment, LED array 260 may include a plurality of channels andeach channel may be driven by an independent driver circuit. Forpurposes of illustration, however, only one channel of LEDs 264 is shownin FIG. 2. In one embodiment, the cathode of each LED 264 iselectrically coupled to one end of a two-terminal switch 266, while theother end of two-terminal switch 266 is coupled to ground 268. Switch266 may be used to select its corresponding LED 264 to be illuminated.

In one embodiment, detection module 240 comprises a multiplexer 242 andan inverter 244 (or NOT gate 244). Multiplexer 242 receives input from aprecision voltage divider 246 and transmits output to the invertinginput (−) of comparator 220. Inverter 244 receives input from a secondterminal of three-terminal switch 230 and transmits output tomultiplexer 242 to hold voltage.

Measurement module 250 includes an inverter 252, an AND gate 254, acounter 256, and a storage unit 258. Inverter 252 receives input from adetect terminal 204 and send the inverted input to a first inputterminal of AND gate 254. A second input terminal of AND gate 254receives an input from terminal 202. AND gate 254 processes the input atthe first and second terminals based on AND logic and transmits theresult to the enable pin of counter 256. In this embodiment, counter 256is a 6-bit counter.

Once enabled, counter 256 begins counting the number of clock cycles inaccordance with a clock signal CLK sent to counter 256 until counter 256is disabled. The resulting number of clock cycles is then transmitted tostorage unit 258 for future uses. In this embodiment, storage unit 258is a static random access memory (SRAM). The enable pin of storage unit258 is electrically coupled to a third terminal of three-terminal switch230.

Three-terminal switch 230 may be switched between a first state and asecond state. In this embodiment, the first state of switch 230constitutes an electrical conduction between the first terminal and thesecond terminal of switch 230, and an electrical insulation between thefirst terminal and the third terminal of switch 230. That is, in thefirst state, a conductive path is formed between comparator 220 anddetection module 240. Likewise, in this embodiment, the second state ofswitch 230 constitutes an electrical conduction between the firstterminal and the third terminal of switch 230, and an electricalinsulation between the first terminal and the second terminal of switch230. That is, in the second state, a conductive path is formed betweencomparator 220 and measurement module 250. Switch 230 may be controlledby a detection signal through terminal 204.

FIG. 3 illustrates a timing diagram for the compensating methodaccording to one embodiment of the present disclosure. As discussedabove, the compensating method includes a preparation step and amodification step. The preparation step further includes a detectionstep, which is illustrated as Part A in FIG. 3, and a measurement step,which is illustrated as Part B in FIG. 3. In one embodiment, thepreparation step may be performed using the circuit illustrated in FIG.2.

Hereafter, the method for driving an LED display panel according to oneembodiment of the present invention will be discussed in more detailwith reference to FIGS. 2 and 3. In Part A, detection of forward voltageV_(f) for a particular LED is performed. In Part B, precisionmeasurement of a time period T_(r) required to reach forward voltageV_(f) is taken.

Part A—Detection of forward voltage V_(f)

In the detection step, three-terminal switch 230 is turned to the firststate. Initially, an LED of LED array 260 is selected. Channel output210 then forces a low current into the selected LED. Timing diagram 310in FIG. 3 illustrates the anode voltage of the selected LED. As shown inPart A of FIG. 3, the anode voltage (or the voltage at non-invertinginput (+) of comparator 220) increases slowly and stabilizes at forwardvoltage V_(f.) In one embodiment, the low current is about 2.0 mA orless, and forward voltage V_(f) is about 2.2 volts.

In this embodiment, voltage multiplexer 242 is used to accuratelymeasure forward voltage V_(f) across the selected LED. Voltagemultiplexer 242 may be programmed by high resolution values in smallsteps to accurately measure voltage variations across different LEDs.That is, voltage multiplexer 242 may be used to increase the voltage atinverting input (−) of comparator 220 in small steps. In one embodiment,voltage multiplexer 242 includes a voltage divider, which may increasevoltage in an increment of 0.1 volts.

When the voltage at inverting input (−) of comparator 220 is equal to orslightly greater than forward voltage V_(f) at non-inverting input (+)of comparator 220, detection module 240 holds that voltage. Timingdiagram 320 in FIG. 3 illustrates the hold voltage obtained throughdetection module 240. Three-terminal switch 230 is turned to the secondstate once the hold voltage is obtained.

Part B—Measurement of time period T_(r)

In Part B, forward voltage V_(f) is measured once again with respect tothe hold voltage obtained in Part A. This time, the measurement focuseson the amount of time (i.e., time period T_(r)) that is required foranode voltage to reach forward voltage V_(f.) Initially, a displaycurrent is applied to the selected LED. In one embodiment, as soon asthe display current is applied, counter 256 is enabled and begins tocount the number of clock cycles. FIG. 3 illustrates a high resolutionclock signal 340 that may be used to take measurement of time periodT_(r).

In response to the display current, anode voltage 310 (or the voltage atnon-inverting input (+) of comparator 220) increases from zero toforward voltage V_(f) and stays at forward voltage V_(f.) Because thevoltage at inverting input (−) of comparator 220 stays at the holdvoltage, comparator output 330 (i.e., the voltage at the first terminalof switch 230) switches from Low to High, once anode voltage 310 reachesor exceeds the hold voltage. As a result, storage unit 258 is enabled tostore the number of counted clock cycles when the output of comparator220 switches. The number of counted clock cycles may correspond to timeperiod T_(r) that is required to reach forward voltage V_(f.) Timeperiod T_(r) may then be used as a compensation parameter during thecompensation phase to modify the PWM driving signal. It is to be notedthat the detection step (Part A) and the measurement step (Part B) maybe repeated until the compensation parameters for all of the LEDs in LEDarray 260 are obtained.

FIG. 4 illustrates a schematic circuit in accordance with one embodimentof the present disclosure. In one embodiment, the modification step maybe performed using the circuit illustrated in FIG. 4. As shown in FIG.4, an LED driving circuit 400 includes an initialization unit 410, a PWMgeneration unit 420, an adding unit 430, an ON/OFF control unit 440, acurrent driver 460, and a constant current controller 450.

Initialization unit 410 may detect forward voltage V_(f) of LEDs andmeasure time period T_(r) for a driving signal to reach forward voltageV_(f.) Time period T_(r) measured by initialization unit 410 may be usedas a compensation parameter to modify a normal PWM driving signal. Inone embodiment, initialization unit 410 may include FCCL circuit 200 asshown in FIG. 2. In one embodiment, time period T_(r) may be measured byperforming the steps in Part A and Part B, described above.

PWM generation unit 420 may generate a pre-determined PWM signal thatmay be used to drive an LED array 470. Adding unit 430 may be used tomodify the pre-determined PWM signal using the compensation parameterdetermined in initialization unit 410. In one embodiment, adding unit430 is used to delay the pre-determined PWM signal using thecompensation parameter. The delayed PWM signal is then sent to ON/OFFcontrol unit 440.

ON/OFF control unit 440 generates a control signal based on the delayedPWM signal and sends the control signal to current driver 460. Constantcurrent controller 450 controls current driver 460 to generate constantcurrents. The constant currents may then be modulated by the controlsignal from ON/OFF control unit 440, thereby generating and delivering aPWM current signal to LED array 470.

Compensation of the PWM driving signal may be performed during thenormal operation of an LED display panel. It is noted that the 6-bitvalue of the measured rise time T_(r) for every LED may be dynamicallyadded to the PWM word as a part of the normal color and brightnesscalibration compensation. When applying the PWM pulse to an LED, thevalue of the measured time period T_(r) may be used as a compensation byadding the value to the tail-end of the PWM driving signal as a delay.Because rise time T_(r) of forward voltage V_(f) may effectively capturethe accumulating effects in an LED display panel, the driving circuitand method of the present disclosure can ensure that equal global chargeis delivered to all LEDs at minimum gray scale settings.

Many modifications and other embodiments of the disclosure will come tothe mind of one skilled in the art having the benefit of the teachingpresented in the forgoing descriptions and the associated drawings. Forexample, the driver circuit can be used to drive an LED array in eithercommon cathode or common anode configuration. Elements in the LED arraycan be single color LEDs or RGB units or any other forms of LEDsavailable. The driver circuit can be scaled up or scaled down to driveLED arrays of various sizes. Multiple driver circuits may be employed todrive a plurality of LED arrays in a LED display system. The componentsin the driver can either be integrated on a single chip or on more thanone chip or on the PCB board. Such variations are within the scope ofthis disclosure. It is to be understood that the disclosure is not to belimited to the specific embodiments disclosed, and that themodifications and embodiments are intended to be included within thescope of the dependent claims.

What is claimed is:
 1. A circuit for ensuring full content cyclelighting of an LED array having a plurality of LEDs, the circuitcomprising: a current source that generates a current signal for theLEDs; a detection module that detects a forward voltage of the LEDs inresponse to a low current applied to the LEDs, the detection modulefurther holds the detected forward voltage; and a measurement modulethat measures a time period for an anode voltage of the LEDs to rise tothe detected forward voltage in response to a display current applied tothe LEDs.
 2. The circuit of claim 1, further comprising: a comparator;and a three-terminal switch electrically coupled to an output of thecomparator, the three-terminal switch being switchable between a firststate and a second state; wherein the first state forms an electricalconduction between the comparator and the detection module, and thesecond state forms an electrical conduction between the comparator andthe measurement module.
 3. The circuit of claim 2, wherein the detectionmodule further comprises a voltage multiplexer configured to transmitvoltage signals in increments to the comparator.
 4. The circuit of claim2, wherein the measurement module further comprises: a counterconfigured to count the number of clock cycles while the anode voltageis not greater than the detected forward voltage, the number of clockcycles corresponding to the measured time period; and a storage unitconfigured to retain the measured time period.
 5. The circuit of claim4, wherein the counter is a six-bit counter and the storage unit is astatic random access memory.
 6. The circuit of claim 1, wherein the lowcurrent is about 2.0 mA or less, and the forward voltage is about 2.2Volts.
 7. A circuit for driving an LED display panel having a pluralityof LEDs, the circuit comprising: an initialization unit; and anoperation unit; wherein the initialization unit comprises: a detectionmodule configured to detect a forward voltage of the LEDs in response toa low current applied to the LEDs, the detection module furtherconfigured to hold the detected forward voltage; and a measurementmodule configured to measure a time period for an anode voltage of theLEDs to rise to the detected forward voltage in response to a displaycurrent applied to the LEDs; and wherein the operation unit comprises: asignal generator configured to generate a driving signal; a compensationmodule configured to modify the driving signal in accordance with themeasured time period; and a current driver configured to receive themodified driving signal and, in response to the modified driving signal,transmit a driving current to the LEDs, so as to illuminate light withdesired brightness.
 8. The circuit of claim 7, wherein the drivingsignal is a pulse width modulation (PWM) voltage signal.
 9. The circuitof claim 8, wherein the compensation module is configured to delay thePWM voltage signal by the measured time period.
 10. An LED displaypanel, comprising a plurality of LEDs and the circuit as recited inclaim 7, wherein the circuit supplies the driving current to the LEDs.